1. Field of the Invention
The present invention relates to an X-ray computed tomography (CT) apparatus acquiring image data on a tomographic plane of a subject, for example, a patient, in synchronism with the subject's respiratory action, and in particular, to an X-ray CT apparatus including a two-dimensional detector detecting cone-beam X-rays emitted by an X-ray source and spreading in the direction of the subject's body axis (cone-beam [CT]).
2. Description of the Related Art
X-ray CT apparatuses are classified into, for example, fan beam (single slice) X-ray CT apparatuses, multi-slice X-ray CT apparatuses, and cone-beam X-ray CT scanners. Scan schemes for the X-ray CT apparatuses include, for example, helical scan and respiratory-gated scan (prospective-gating). Projection data acquired by helical scan is reconstructed by, for example, respiratory-gated reconstruction (retrospective-gating).
The fan beam X-ray CT operates as follows. The X-ray source emits fan-like X-ray beams. A plurality of detectors are arranged in a line in fan form so as to provide, for example, nearly 1,000 channels. The dose of X-rays having passed through a subject is detected by the plurality of detectors. The dose of X-rays detected by the plurality of detectors is digitally converted and collected by a data collection circuit. The data collection is performed about 1,000 times during one revolution of the X-ray source and the plurality of detectors revolved around the subject. Images are reconstructed based on the collected data.
The multi-slice X-ray CT includes an X-ray source emitting conical X-ray beams and a two-dimensional detector. The two-dimensional detector is made up of a plurality of detection elements arranged on a cylindrical plane so that N fan beam detection rows each of a plurality of (M) detection elements are stacked in the direction of a Z-axis (the direction of the subject's body axis). For the two-dimensional detector, the numbers of channels and segments are defined as M and N, respectively. The distance between the focus and the center of rotation and the diameter of an effective visual field are defined as FCD and FOV, respectively.
In the cone-beam X-ray CT scanner, the X-ray source emits wider X-ray beams in the direction of the subject's body axis. In the cone-beam X-ray CT scanner, data detected by the two-dimensional detector and collected from one direction corresponds to two-dimensional projection data. The cone-beam X-ray CT scanner performs three-dimensional image reconstruction based on two-dimensional projection data from multiple directions. The cone-beam X-ray CT scanner acquires voxel data on a certain volume simply by allowing the X-ray source and the two-dimensional detector to make one revolution.
For the cone-beam X-ray CT scanner, since the latter half of 1980s, research and development efforts has focused mainly on a system using an X-ray image intensifier as a two-dimensional detector. For example, the document “Volume CT of anthropomorphic phantoms using a radiation therapy simulator Michael D. Silver, Yasuo Saito, et al., SPIE 1651, 197-211, 1992” describes discussions on the results of scan of anthropomorphic phantoms using an experimental system that is a combination of a turntable and an image intensifier. The cone-beam X-ray CT scanner using the image intensifier has been used in some applications in order to capture the shape of an object offering a high contrast, for example, a bone or an imaged blood vessel.
A proposed cone-beam X-ray CT scanner includes scintillator similar to the multi-slice CT and photodiodes serving as detection elements. The proposed cone-beam X-ray CT scanner is combined with a continuously rotatable slip ring frame. The cone-beam X-ray CT scanner is described in, for example, the document “Large area 2-dimensional detector for real-time 3-dimensional CT (4D-CT), Yasuo Saito et al., SPIE 4320, 775-782, 2001”.
An X-ray CT scanner using helical scan continuously revolves an X-ray tube and a detector around the subject, while moving a bed top plate with the subjected lying thereon, in the direction of the subject's body axis. Thus, the X-ray tube follows a spiral track relative to the subject.
To reconstruct an image, the X-ray CT scanner performs scan at a timing for a preset phase of respiratory movement obtained using a device monitoring the subject's respiratory action to collect data corresponding to a certain portion of the track of the X-ray tube. The monitoring device monitoring the subject's respiratory action may be, for example, a pressure sensor attached to the subject's chest, an air flow sensor measuring the flow rate of the subject's breath, or a device determining the respiratory movement by allowing software to analyze the subject's motion the image of which is captured by a camera.
The X-ray CT scanner uses a 360° interpolation method or a 180° interpolation method. The 360° interpolation method uses data for 180° on the opposite sides of a slice plane. The 180° interpolation method uses data for a total of 180° across the slice plane which is obtained using opposite beams.
The respiratory-gated scan adjusts scan timings in synchronism with the subject's breath during the scan. The monitoring device monitors the subject's respiratory action. The monitoring device may be, for example, a pressure sensor attached to the subject's chest, an air flow sensor measuring the flow rate of the subject's breath, or a device determining the respiratory movement by allowing software to analyze the subject's motion the image of which is captured by a camera, as described above. The respiratory-gated scan is performed at a timing for a preset phase of the subject's respiratory movement monitored by the monitoring device.
A technique is available for acquiring images of the same respiratory phase over a broad range in the direction of the subject's body axis. The technique repeats performing a scan in which the X-ray tube and the detector make one revolution around the subject at rest and moving the subject.
Specifically, first, the subject is moved in order to allow an image of a required slice position of the subject to be acquired. In this condition, the scan is performed at a timing for a preset respiratory phase. Then, the subject is moved in order to allow an image of the next required position of the subject to be acquired. The subject waits for the scan to be performed at the timing for the preset respiratory phase. If such a scan is performed to acquire volume data at different respiratory phases, the above-described series of operations are repeated at different respiratory phases.
In the respiratory-gated reconstruction, a postprocess is executed in which projection data acquired by the helical scan and which is consecutive in the directions of time and the subject's body axis is reconstructed into volume data for any respiratory phase. The monitoring device monitoring the subject's respiratory action may be, for example, a pressure sensor attached to the subject's chest, an air flow sensor measuring the flow rate of the subject's breath, or a device determining the respiratory action by allowing software to analyze the subject's motion the image of which is captured by a camera, as described above. The monitoring device monitoring the subject's respiratory action monitors the subject's respiratory action to output respiratory monitoring signals, for example, gate pulses or waveforms.
To perform the respiratory-gated reconstruction, the respiratory monitoring signals output by the monitoring device are stored together with the projection data. In the respiratory-gated reconstruction, data required to reconstruct the respiratory phase specified based on the respiratory monitoring signal is extracted from the projection data, to reconstruct an image covering the specified range. To allow an image for any position and any respiratory phase to be reconstructed, data on the same slice position needs to be collected for one cycle of respiratory time interval. Thus, the range of helical pitch is limited by the respiratory cycle, the number of detector rows, and the like.
However, one respiratory-gate scan allows volume data to be obtained for only one particular type of respiratory phase. Thus, images involving the subject's motion associated with the respiratory movement cannot be observed. By repeating the scan with the specified respiratory phase varied, volume data can be obtained for some respiratory phases. However, this is still insufficient for observing the subject's consecutive motions associated with the respiratory movement.
The respiratory-gated reconstruction enables the subject's motion to be observed under the precondition that the same breath is repeated. However, because of the precondition that the same breath is repeated, the respiratory-gated reconstruction is limited to resting respiration. Thus, the respiratory-gated reconstruction fails to enable diagnostic functional analysis based on motion associated with a conscious deep breath, for example, the motion between the maximal expiration and the maximal inspiration. For example, during the subject's maximal expiration or maximal inspiration, images of a site that cannot be observed at a resting respiratory level, for example, images of a tumor site, can be observed.
However, the respiratory-gated reconstruction is limited to the resting respiration, and thus has difficulty acquiring images during, for example, the subject's maximal expiration or maximal inspiration.
Such a cone-beam X-ray CT scanner is described in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2001-242253.
An object of the present invention is to provide an X-ray CT apparatus allowing volume data to be obtained for any respiratory phase of the subject's motion associated with deep respiratory movement, even during a non-reproducible breath such as a conscious deep breath.